Note: Descriptions are shown in the official language in which they were submitted.
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Title: "Method of Repair of Turbines"
The present invention relates to a method of repair of turbines and parts
thereof and is primarily concerned with the repair of stator blades sometimes
known as nozzles or diaphragms, the purpose of which is to direct steam or gasesonto the rotor blades.
In use of a turbine, stator blades situated at the high pressure end of the
turbine are subject to wear caused primarily by particles carried by the steam,
most of the wear occurring on the face which is directed towards the incoming
steam. However, the reverse face of the stator blade is also subjected to wear
due to the effect of steam or particles rebounding from the adjacent row of rotor
blades onto the reverse face of the stator blades.
In the past where wear or damage of sufficient magnitude to impair the
efficiency of a turbine had occurred, the stator blades were replaced which
inevitably led to a very substantial expense.
It has been proposed to minimi~e such wear by providing on the trailing
edges of the stator blades, the area most susceptible to damage, a Stellite insert.
Due to the incompatibility of stellite with the parent material of the blade, usually
a 12~o chrome steel, a relatively soft weld material Inconel had to be used in
order to ensure that the insert was properly secured to the stator blade and in
view of the inclement conditions inside a turbine, the Inconel was subject to a
much greater wear rate than the stator blade or of course the Stellite insert.
In parts of a turbine, e.g. the first set of blades of the high pressure stage
or intermediate pressure stage, the stator blades are situate very close to eachother m~king repair thereof extremely difficult. Whereas the worn trailing edge
could in some cases be built up by several weld passes this meant all the new
material put onto the blade was "cast~ and in view of the number of passes
needed the heat input to each stator blade was considerable which could lead to
distortion which was subsequently very difficult, if not impossible to correct thus
leading to uneven gaps between the stator blades and the chance of considerable
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inbuilt stress into the structure as a whole which again could lead to prematurefailure.
It is an object of the present invention to provide a method of repairing
stator blades that overcomes or minimi~es the problems mentioned above.
According to one aspect of the present invention we provide a method of
repairing stator blades co~ lg the steps of:-
1. preparing a pre-shaped insert made from a material the same as or
compatible with said stator blade, said insert affording at least a
substantial part of the of said stator blade;
2. treating said insert so that at least a part thereof has a surface that
is harder than the material from which the insert is made;
3. removing a substantial part of said stator blade, the shape of the
part removed being sufficient to accommodate said insert;
4. welding said insert to said stator blade with a weld material the
same as or similar to the stator blade and/or insert material.
Preferably, said substantial part is large enough to ensure that any weld
line extending substantially parallel to the trailing edge is sufficiently far from the
trailing edge to be away from any area of high wear.
The insert for example may comprise 20% of the surface area of the blade
and in some cases may be greater.
Preferably, said insert comprises a substantial part of the trailing edge of
said stator blade. Alternatively said insert comprises a substantial part of said
leading edge.
Preferably said method of repair may also include heat treatment processes
to provide stress relief to the stator blade after repair thereof.
Since the material of the insert is the same as or compatible with the
material of the turbine blade and preferably the securing of the insert to the
blade may be made with a similar material and it is not necessary to use a soft
or absorbent weld material which, even though such material may provide a
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satisfactory bond between the parts, because of its softness it is prone to erosion
which may lead to the insert becoming detached from the remainder of the stator.Whereas it is envisaged that the material of the insert may be the same as
or very similar to the material of the blade, it is also envisaged that a hardermaterial may be used which, even though harder, may still be compatible with theturbine blade in that it may be secured thereto using a weld material that is
relatively hard.
Where a different material is used, such a material may be chosen for its
enhanced wear characteristics or perhaps its suitability for hardening treatments
such~ as diffusion alloying or plasma spray and it is envisaged that the insert may
not orlly be treated before ~tt~hment to the remainder of the stator blade but
further hardening and/or coating treatments may be carried out after it has beensecured thereto. For example it is envisaged that an insert may be hardened by
a diffusion alloying process and after securing to the remainder of the stator blade
may be subjected to plasma spray to provide a further wear resistance coating.
If desired, the area of the weld may also be subjected to plasma spray to
provide a capping to prevent wear of the weld material or alternatively a Stellite
cap may be provided over the weld.
It is also envisaged that a plurality of coatings of plasma spray may be used
to build up the repaired stator blade to a predetermined desired shape and a
plurality of passes may be used so as to build up some areas more than others toproduce the required shape of blade to prolong its life or efficiency or both.
It is envisaged that in some cases, the wear that has occurred to turbine
stator blades may not in itself necessitate repair. However, the method of repair
is equally applicable when it is desired to make slight alterations to the shape of
the stator blades so as to improve efficiency.
During the manufacture of a stator array that is a diaphragm rather than
a nozzle array, it is usual for the blades to be held in a mould and inner and
outer rings are then formed by a casting process which sometimes disturbs the
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relative positions of the blades. The stator blades may then have different gapsbetween different blades and/or individual blades may be slightly mi~ligned.
The positioning and shape of a stator blade is highly critical the
performance of a turbine and even slight errors may lead to a considerable drop
in efficiency and hence to a significantly increased fuel cost to generate a certain
amount of electricity.
It is envisaged that the repair method of the present invention may be used
so as to ensure that all the stator blades are correctly positioned relative to each
other and each stator blade has the correct configuration and form so as to ensure
m~ximllm efficiency of at least that part of the turbine.
The positioning of the inserts which may form a substantial part of each
stator blade may be facilitated by the use of guides locatable on a first statorblade to pre-set the position of an insert for an adjacent stator blade prior towelding of that insert to the remainder of the stator blade.
Since relative movement between the insert and the remainder of the
stator blade may be expected to occur during welding, guides which are used to
position the inserts but not to constrain their movement may intentionally position
the insert in an out-of-alignment condition and, when welding has been
completed, the outer position ~ligning of the insert may be compensated by the
movement occurring between the insert and the remainder of the stator blade
during welding.
The positioning of the joint away from the trailing edge of the blade and
also away from any high wear area, not only moves the area of possible weakness,i.e. the weld, to an area far less likely to be prone to erosion during use of the
turbine but also ensures that a large piece of the stator blade is removed, hence
permitting easier access to an adjacent blade and treatment thereof by plasma
deposition for example or during the welding process.
Since plasma deposition is a "line of sight" process, unless a substantial part
of the insert is removed, then it would not be possible to carry out plasma
deposition on much of the stator blade since access thereto would be restricted
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by ~dj~cent stator blades, the removal of the substantial part enables such access
and hence allows a stator blade to be built up using plasma deposition so that the
final shape and form may be accurately controlled.
Whereas it is envisaged that each insert will be hardened prior to securing
to the remainder of the stator blade, with the exception of an area along which
welding is to take place, such treatment to produce a hardened surface or
hardened area may be selected and, when a stator blade array to be repaired is
inspected and the characteristics of the wear can be seen, it is envisaged that
selective hardening of the inserts may take place so as to ensure that the areasthat appear more prone to erosion are best treated in a manner to minimice
further occurrence of such erosion and hence increase the life expectancy of thearray.
It is also envisaged that after inspection of a worn array, changes may be
made to the design of each blade as a whole by inserting an insert of different
thickness, shape or configuration so as not only to increase efficiency of the
turbine as a whole but also by modifying the shape to reduce erosion.
Since the inserts are treated prior to securing to the remainder of the array
and it is envisaged that diffusion alloying may be a preferred form of treatment,
such treatments require the insert to be elevated to very high temperatures which,
if carried out on the array as a whole, could cause considerable damage to otherparts of the array leading to inbuilt stress not capable of satisfactory stress relief
and could disturb the whole assembly.
The treatment of the inserts in isolation with any m~king necessary for
weld areas ensures that a satisfactory hardening process can take place without
any adverse effects to other parts of the array.
In some cases, it may be that damage has not only occurred to each stator
blade but also to the inner and outer rings between which the blades are securedand the method of repair or modification of the present invention enables, afterremoval of a substantial part of each blade to be repaired or modified, access to
the inner circumferential surface of the outer ring and the outer circumferential
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surface of the inner ring since access is permitted by removal of a substantial part
of each blade, this enables repair to be carried out to such surfaces, for example
by the build up of material by plasma deposition or by the insertion of a thin
sheet of hard material which may be secured thereto in any suitable manner and,
because of its hardness, provide sati~f~ctory resistance to further erosion.
It is normal for stator blades, at least in steam turbines, to be made from
a 12Yo chrome steel. However, the exact make up of this chrome steel may alter,
depending upon whether they are nozzles or diaphragms and whether they are
intended to be situated in the hight pressure section of the turbine, the
intermediate pressure or the low pressure.
The method of the present invention envisages that blades in any section
in a turbine may be made from a composite material and, for example, a
diaphragm situated in the low pressure area may have an insert made from a
material more usually used in a high pressure section of the turbine, the insertwill be compatible with the material from which the remainder of the low
pressure diaphragm is made but offering enhanced wear characteristics.
The hardening of said insert may comprise any suitable hardening process
such as diffusion alloying, a plasma spray process, heat hardening or any other
suitable process to ensure that at least the edge region of the blade insert has a
hard and hence wear resistant surface.
Preferably at least the one side, e.g. the pressure side, of the blade is
subjected to hardening by diffusion alloy and before or after securing of the insert
to the remainder of the blade, the other side of the blade may be subjected to
hardening by plasma spray and if required the said one side may also be subjected
to a plasma spray process.
Preferably the substantial part of the blade that is removed is a larger part
than would appear necessary to repair wear or damage caused to the blade which,
even though it would appear unnecessary, gives significant and somewhat
unexpected advantages:-
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1. the removal of a large part of each blade enables access to adjacentblades to allow the repair process to be carried out, which repair
process would not have been possible without removal of a
substantial part of each blade;
2. by ensuring that each insert comprises a substantial part of each
blade, it is ensured that in normal circumstances a plurality of
inserts of exactly the same size may be produced and which will be
sufficiently large to ensure that all damaged parts of each blade are
replaced by new material;
3. since the blade insert comprises the trailing edge region, which is
a relatively thin part of the blade, by providing an insert of
substantial size the material of the insert as it approaches the
leading edge of the blade will be substantially thicker than the
trailing edge thereby ensuring that the insert is joined to the
remainder of the blade over at least the length thereof having a
substantial thickness thereby m~king a much more secure joint;
4. the majority of the joint line between the insert and the blade will
be well spaced from the trailing edge ensuring that any weakness
afforded by the weld line is spaced well away from the area of
maxlmum wear.
It is normal when such an insert for movement of the insert or remainder
of the blade to take place due to the application of considerable heat during the
welding process.
Once the expected movement is known it is envisaged that the insert may
be apparently misplaced before welding to the blade so that the movement that
is expected to occur during welding will return the blade and/or insert to its
intended position.
The shape and positioning of blades, particularly in steam turbines, is
highly critical to their efficiency and the repair process of the present invention
may be used to improve the shape of existing blades without necessitating their
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complete replacement and/or h~ rove the accuracy of the positioning of each
individual blade and the spacing between adjacent blades so as to further improve
the efficiency of the turbine and may be carried out to turbine blades for the sake
of improvement of efficiency even though individual blades may not be
particularly worn or damaged.
The repair method of the present invention has a further advantage over
and above replacement of the blades in that it~considerably decreases the time
necessary to fully repair a blade array, particularly since all the inserts being of
identical shape may be pre-formed and hardened and since an identical piece is
removed from each blade, this permits of some mech~ni~Ation or autornisation of
the process for removing a piece of each blade.
The invention will now be described by way of example only with reference
to the accompanying drawings, wherein:-
FIGURE 1 is a view of a turbine stator blade array;
FIGURE 2 is a view of part of the array shown in Figure 1 showingdamage to the turbine blades;
FIGURE 3 is a view similar to Figure 2 after removal of part of each
blade;
FIGURE 4 illustrates an insert;
FIGURE S is a view of the stator blades after the inserts have been
welded thereto;
FIGURE 6 is a view of the ~mished repaired stator blade array;
FIGURE 7 is a section through a stator blade.
Referring first to Figure 1 the stator blade array comprises an outer
~nn~ r component 10, an irmer ~nmll~r component 11 between which are
secured a plurality of blades such as those shown at 12, 13 and 14.
The blade array shown in Figure 1 shows the trailing edges of the blades
which are those most prone to wear during use.
The blade arrays are normally formed in two halves and the array may be
separated along the lines shown at lS and 16.
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Referring now to Figure 2, the trailing edges of the blades 12, 13, 14 to
which wear and damage has taken place, as shown by damaged edges 17, 18 and
19.
In order to carry out a repair, pre-formed inserts or coupons are formed,
such as that shown in Figure 4 and illustrated generally at 20. The coupon may
be of any desired shape depending upon the dimensions and shape of blade to be
repaired but typically is of a shape in which the insert forms a very substantial
part if not all of the trailing edge region 21 and has a dimension transverse to the
trailing edge 21 which comprises a substantial part of the stator blade, for
example 20~o and may in some cases approach 50% of the whole or even greater.
Such a dimension ensures that the region as indicated by the arrow 22 is well
spaced from the trailing edge region 21 which is usually more prone to damage
and wear than the leading edge of the blade.
The insert 20 is formed from a material the same as or at least compatible
with, i.e. sirnilar hardness and similar material, the parent material of each stator
blade and then is subjected to some suitable form of hardening process in which
a substantial part if not all of the insert 20 has at least a hard surface area.If desired an edge region on the semi-circular weld line 23 may be masked
from such hardening process to ensure that the material along the weld line 23
is substantially unaffected by the hardening process.
Referring now in addition to Figure 3, a piece of each blade is removed
such as on blades 12, 13 and 14 to leave a cut-out 24, 25 and 26 of a shape to
accommodate an insert such as that shown in Figure 4.
Since the blade array as shown in Figure 1 is formed in two halves, the
array may be separated and an end stator blade is first machined to remove the
cut-out such as that shown at 24, 25, 26 and then a cut-out may be made on each
blade in turn, the cut-out made in the adjacent blade previously enabling accessto the next to make the necessary cut-out.
After the necessary pieces have been cut out from each blade, the insert
such as that shown at 20 in Figure 4 may be secured in place by welding and
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Figure S illustrates the array in which a plurality of inserts such as that shown at
20 in Figure 4 have been secured in place, such as the inserts 28, 29 and 30 on
stator blades 12, 13 and 14.
Whereas in the embodiment illustrated the inserts are of a shape to
replace pieces of material of the same dimension it is envisaged that an insert
may extend the boundaries of each stator blade and may be of a shape different
from the original shape of the blade so as to improve efficiency of the turbine or
improve other operating characteristics to enhance the working of the turbine asa whole or its life expectancy.
After welding of each insert to the blade, using a weld material that is
similar to the material of the insert and/or blade which in all events are similar
or compatible with each other, any work necessary on the weld filet may be
carried out to return the stator blade array to its original form as shown in Figure
6.
It is envisaged that the weld may be a penetration weld carried out from
one side only of each blade, the cut-out in the adjacent blade permitting of
sufficient access to the blade to carry out a satisfactory weld process. If the part
of the blade that has been removed was not of sufficient size, such access wouldnot be possible.
A further advantage gained in carrying out the weld from one side only in
that the side where most weld will be visible may be selected to be that side
which is less prone to wear during use of the turbine.
A heat treatment processes may be carried out for stress relief purposes
during which the blades may be raised to predetermined temperatures for a
predetermined length of time and maintained for a predetermined length of time
at such temperatures after which they may be allowed to cool in a controlled
manner.
Figure 7 is a section through a stator blade 40 which has a high pressure
side 41, a low pressure side 42 and leading edge region 43, and a trailing edge
region 44.
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Since it is the high pressure side 41 that is most prone to erosion and
damage, it is envisaged that at least the high pressure side 41 of a new insert will
be subjected to diffusion alloying to produce a very hard surface finish and theside 42, the vacuum side or low pressure side may be subjected to a plasma sprayprocess after the insert has been secured to the blade.
It is further envisaged that the high pressure side 41 in addition to the
diffusion alloying may be subjected to a plasma spray process after securing to the
blade.
The features disclosed in the foregoing description, or the accompanying
drawings, expressed in their specific forms or in the terms or means for
performing the desired function, or a method or process for attaining the
disclosed result, may, separately or in any combination of such features, be
utilised for re~lising the invention in diverse forms thereof.
